The present invention relates to a rubber composition for a tire and a pneumatic tire, particularly a rubber composition for a tire that has excellent friction on ice. More specifically, the present invention relates to a rubber composition for a tire, wherein friction on icy and snowy road surfaces is improved and the filler is prevented from dropping out when running the tire. Thus, a filler having at least two protuberances is compounded in diene rubber, and the composition is used in a tire tread of a pneumatic tire.
Conventionally, when driving automobiles on icy and snowy road surfaces, spiked tires are used or chains are fixed to the tires. However, because of environmental problems such as the generation of dust which occurs, using the approach, a studless tire has been developed, as an alternative tire, for running on icy and snowy road surfaces.
Icy and snowy road surfaces are slippery, because the friction coefficient is significantly lower than normal roads. Therefore, various attempts have been made to improve studless tires, from the viewpoints of material and design. For example, in order to improve friction on ice, known are the methods, of using a rubber composition containing diene rubber having excellent properties in low temperatures; the method of increasing the surface edge component by changing the unevenness of the tire surface; and the method of obtaining digging effect to ice on icy and snowy road surfaces by compounding short fiber, natural glass or an inorganic filler into a rubber composition (see JP-A-2002-114868, JP-A-2001-39104, JP-A-2002-53704 and JP-A-8-217918).
However, there is the problem that the scratching effect is lost, since the compounded material falls out due to stimulation and abrasion when running, and thus a studless tire is still insufficient in friction on icy and snowy road surfaces compared to a spiked tire. Consequently, further improvement is required.